A great many IMDs for cardiac monitoring and/or therapy comprising sensors located in a blood vessel or heart chamber coupled with an implantable monitor or therapy delivery device have been proposed or implemented. For example, such cardiac IMDs include implantable heart monitors and therapy delivery devices including pacemakers, cardioverter/defibrillators, cardiomyostimulators, ischemia treatment devices, and cardiac drug delivery devices. All of these systems include electrodes for sensing and sense amplifiers for recording and/or deriving sensed event signals from the intracardiac electrogram (EGM). In current implantable cardiac devices providing a therapy the sensed event signals are utilized to control the delivery of the therapy in accordance with an operating algorithm and at least selected EGM signal segments and sensed event histogram data or the like are stored in internal RAM for telemetry out to an external programmer in real time or at a later time.
Efforts have also been underway for many years to develop implantable physiologic signal transducers and sensors for temporary or chronic use in a body organ or vessel usable with such IMDs for monitoring a physiologic condition other than or in addition to the EGM to derive and store data and/or to control a therapy delivered by the IMD. A comprehensive listing of implantable therapy delivery devices are disclosed in conjunction with implantable sensors for sensing a wide variety of cardiac physiologic signals in U.S. Pat. No. 5,330,505, incorporated herein by reference.
Blood pressure signal values respond to changes in cardiac output that may be caused by a cardiac failure, e.g., fibrillation or high rate tachycardia, or that may reflect a change in the body's need for oxygenated blood. In the former case, monitoring of a substantial drop in blood pressure in a heart chamber, particularly the right ventricle, alone or in conjunction with an accelerated or chaotic EGM, was proposed more than thirty years ago as an indicia of fibrillation or tachycardia that could be used to trigger automatic delivery of defibrillation or cardioversion shock. More recently, it has been proposed to monitor the rate of change in blood pressure (dP/dt) that accompany normal heart contraction and relaxation and blood pressure changes that occur during high rate tachycardia and fibrillation or flutter.
A number of cardiac pacing systems and algorithms for processing the monitored mean blood pressure or monitored dP/dt have been proposed and, in some instances employed clinically, for treating bradycardia. Such systems and algorithms are designed to sense and respond to mean or dP/dt changes in blood pressure to change the cardiac pacing rate in a rate range between an upper and a lower pacing rate limit in order to control cardiac output. adjoining blood vessels and heart chambers during the cardiac cycle, blood temperature, blood pH, and a variety of blood gases. Such Implantable hemodynamic monitors and blood pressure and temperature sensors that derive absolute blood pressure signals and temperature signals are disclosed in commonly assigned U.S. Pat. Nos. 5,368,040, 5,535,752 and 5,564,434, and in U.S. Pat. No. 4,791,931, all incorporated by reference herein. The MEDTRONIC.RTM. Chronicle.TM. Implantable Hemodynamic Monitor (IHM) employs the leads and circuitry disclosed in the above-incorporated, commonly assigned, '752 and '434 patents to record the EGM and absolute blood pressure values for certain intervals. The recorded data is periodically transmitted to a programmer operated by the physician in an uplink telemetry transmission from the IHM during a telemetry session initiated by a downlink telemetry transmission from the programmer's radio frequency (RF) head and receipt of an interrogation command by the IHM.
Certain of the measured physiologic signals derived from the heart or blood in the circulatory system are affected by ambient conditions that cannot be separately measured by the above-described IMDs and physiologic sensors. Specifically, blood pressure signal values derived by a wholly implantable system, e.g., the IHM described above are affected by atmospheric pressure acting on the patient. Blood temperature signal values derived by a wholly implantable system, e.g., that disclosed in the above-referenced '752 and '434 patents, are affected by ambient temperature or by a fever afflicting the patient, respectively.
Changes in ambient conditions other than weather changes can also influence the measurement of absolute blood pressure changes, including both mean or average blood pressure and dP/dt, by implantable pressure sensors. For example, when a patient in which such an implantable blood pressure sensing medical device is implanted changes elevation by ascending or descending in an elevator in a tall building or in an airplane, the change in barometric pressure mask changes that are sought to be measured. In the context of an implantable rate responsive pacemaker operating under a rate control algorithm, the pressure change caused by the elevation change itself may exceed the blood pressure change that reflects a change in exercise level of the patient and be misinterpreted as meriting a change in pacing rate to the upper or lower pacing rate limit, which can, at least, be uncomfortable to the patient. The barometric pressure effect can similarly have a negative effect on operating and detection functions of other IMDs reliant on accurately sensing cardiac blood pressure changes that truly reflect a cardiac function or requirement for cardiac output.
The deleterious effect of barometric pressure on cardiac blood pressure measurement has been noted. In commonly assigned U.S. Pat. No. 4,407,296, a micro-machined pressure sensor is disposed at the distal end of a lead in an oil filled chamber on one side of a pressure sensor element that is closed by a flexible membrane. The membrane is disposed behind a protective grill at the distal tip of the lead within which blood fluids can contact the exposed side of the membrane. Blood pressure changes deflect the membrane, and the deflection is transmitted through the oil to the micro-machined pressure sensor element which is deflected to produce a pressure signal value change proportional to the blood pressure change acting on the membrane. The blood pressure change reflects both the blood pumping action of the heart and the ambient atmospheric pressure acting on the patient's body. In a first embodiment, the effect of atmospheric pressure is attempted to be offset by providing a chamber behind the sensor element that is sealed at a known average atmospheric pressure. In practice, this approach has proven to be inadequate because the known pressure cannot account for changes in barometric pressure and renders the blood pressure measurements ambiguous.
In a second embodiment, the chamber behind the sensor element is filled with oil and extends proximally through a lumen of the lead body to a further positioned in the subcutaneous cavity under the patient's skin where the implantable monitor or pulse generator is implanted. In this case, the membrane on the lead body is difficult to manufacture, fragile and can become obstructed in chronic implantation. Moreover, the oil filled lumen can be generally either vertical or horizontal in all or in part depending on a number of factors, including the implantation path of the lead body between the subcutaneous cavity and the implantation site of the pressure sensor in the patient's heart chamber and whether the patient is upright or supine. The force that the oil in the oil filled lumen applies to the pressure sensor element depends on the orientation of the lumen with respect to the force of gravity which can change as the patient changes posture. Therefore, the force of the oil column biases the pressure sensor element in an unpredicatable manner, and the reference pressure varies unpredictably and may not represent barometric pressure.
In recognition of these problems with absolute pressure sensors employed to measure blood pressure in a heart chamber or blood vessel, it is suggested in the above-incorporated, commonly assigned, '752 and '434 patents that the patient be provided with a belt worn, external pressure recorder that records and time stamps recordings of barometric pressure that can be retrieved and used for comparison with the internally recorded absolute blood pressure data. Such an external barometric pressure recorder is intended to be used with the above-referenced MEDTRONIC.RTM. Chronicle.TM. IHM. The barometric pressure reference values that are periodically stored in the memory of the external recorder are read out at the time that the absolute pressure data is telemetered out. The reference values are subtracted from the absolute values to derive the relative pressure values.
The barometric pressure sensor is contained in an air chamber within the housing of the external pressure recorder. The air pressure in the air chamber is expected to equalize with atmospheric pressure by air passage through seams that remain when the housing halves are assembled together. Moisture, liquids, can accumulate in the air chamber. For example, the patient might get caught in a rainstorm or splash water on or drop the pressure sensor module into water, allowing water to contact the barometric pressure sensor, and dust can enter the seams and accumulate on the barometric pressure sensor. These contaminants can alter the barometric pressure sensor response to atmospheric air pressure, resulting in the storage of corrupted barometric pressure values which are not possible to detect. The corrupted reference pressure values can therefore contribute to erroneous relative cardiac pressure values when the stored reference pressure values are subtracted from the telemetered out absolute pressure values.
Despite the considerable effort that has been expended in designing such IMDs and associated sensors for sensing such physiologic signals, a need exists for a system and method for accurately accounting for ambient conditions surrounding the patient that affect the sensed and measured physiologic signal values, particularly in the case of blood pressure.